A trench-gate metal oxide semiconductor (TMOS) field effect transistor is a semiconductor device whose gate structure is embedded in the etch trench of the epitaxial layer. Since the carrier drift path of such field effect transistor is formed along the sidewall of the trench, the channel length of the field effect transistor can be largely increased and the resistance of the characteristic channel can be largely reduced by about 30%. Therefore, if the operating current is constant, the static power loss is reduced and the device current density is increased. As known, the conventional planar channel field effect transistor fails to simultaneously increase device current density and reduce the on-resistance. However, the trench-gate metal oxide semiconductor field effect transistor can achieve these purposes. As the feature size and the wiring space are gradually reduced, the trench-gate metal oxide semiconductor field effect transistor becomes more important.
However, since the integrated circuit becomes more and more complicated, the development of the trench-gate metal oxide semiconductor field effect transistor is limited. For complying with the demands on the increasing integration and diversified functions of the integrated circuit and reducing the fabricating cost, there is a need of integrating the structures and the producing processes of the trench-gate metal oxide semiconductor device and the planar channel metal oxide semiconductor device.